The present disclosure relates generally to an epoxy resin composition for manufacturing composite parts and, more particularly, to a curing component that increases the thermal, mechanical and chemical properties of cured epoxy and epoxy composite parts.
Polymer composites offer several advantages compared to metals and ceramics in that polymer composites are lightweight, have high specific stiffness and strength, are easy to manufacture, allow tailoring of the properties by varying the resin's chemistry, reinforcement fibers, and design flexibility for different applications and also have low coefficients of thermal expansion.
Polymer composites, in particular thermosetting polymer material, prepared through crosslinking reaction with an appropriate curing agent, based on amine or polyamine(s), and with an epoxy resin desirably have the following properties: (a) low to high curing temperature, where the curing reaction can be carried out at a temperature of 5 to 150° C.; (b) low volume shrinkage rate, where the volume shrinkage rate of cured epoxy resin is typically 1 to 3% resulting in low internal stress in fiber matrix composites; (c) good wetting, adhesion to provide good shear strength between fiber and matrix; (d) good insulating properties; (e) good chemical resistance; and (f) good thermal properties.
The properties of cured epoxy systems allow their use in various fields and they are extensively used in industry such as adhesive, coating, and composite applications.
Many composite applications require a high degree of resistance to heat. Epoxy amine systems develop relative high glass transition temperature (Tg) (about 150° C.) when cured at high temperature (about 150° C.). However, higher Tg (greater than 170° C.) is desired for many applications to successfully replace heavy metals and metal alloys. To achieve such high Tg, extensive crosslinking is required, which usually tends to cause embrittlement.
Typically, high Tg will usually provide a high chemical resistance, but not with all chemicals. For example, acetone and methanol can disturb the crosslinked network (e.g., penetration of the matrix to the fibers).
Aromatic, cycloaliphatic, aliphatic, heterocyclic and polyether polyamines have been used in the past for the curing of epoxy resins. It is well known that aromatic amines provide better chemical resistance and thermal stability than cycloaliphatic and aliphatic or other polyamines. Several approaches have been considered to increase the glass transition temperature and chemical resistance of epoxy formulations. For structural applications it is desirable, particularly for reinforced composites, to produce an epoxy having a high Tg, improved chemical resistance, while maintaining the mechanical properties for overall composite performance. Although higher Tg can be achieved by formulating epoxy resins such as bisphenol-A (BPA), bisphenol-F (BPF) with epoxy novolac. The main drawback with this approach is a large increase in the viscosity of the formulation. High viscosity leads to processing challenges and increases flaws in the composite matrix due to the resin's very high viscosity. Numerous technical challenges exist to increasing the Tg and chemical resistance without sacrificing other properties.
Accelerators are generally known for use to accelerate curing agents which have very slow reactivity with epoxy resins (e.g., dicyanamide (DICY), anhydride, and aromatic amines). Epoxy resin systems are not generally known to be formulated to increase the Tg of liquid curing agents epoxy system. Known epoxy systems having accelerator compounds included in the formulation are single component (1K) systems, typically found in adhesive and powder coating where they use the accelerator to increase the reactivity of amine in the latent epoxy system.
U.S. Pat. No. 5,512,372, which is hereby incorporated by reference in its entirety, discloses a hardenable, or polymerizable epoxy resin-based composition, characterized in that its hardener system comprises a combination in synergic quantities of at least one imidazole which, alone, can cause the polymerization of the epoxy resin only at a temperature above about 75° C. The U.S. Pat. No. 5,512,372 shows the use of imidazole as accelerator for one component polyamines, such as dicyanamide (DICY), and modified polyamine ANCAMINE® 2014 and does not disclose any thermal, mechanical or chemical resistance improvements. The U.S. Pat. No. 5,512,372 discloses one component systems and is specific to the latent curing agents. Further, the U.S. Pat. No. 5,512,372 only discloses a one component system having polyamine which is latent at room temperature for more than 21 days and none of the examples disclose amines that are active at room temperature with epoxy resins. The one component system disclosed in the U.S. Pat. No. 5,512,372 includes high loadings of imidazole and undesirably low Tg for the cured formulation. In addition, the U.S. Pat. No. 5,512,372 does not disclose compositional ranges that include off-stoichiometric ratios of curing agent to epoxy.
International Patent Application Publication WO2009/089145, which is hereby incorporated by reference in its entirety, discloses an epoxy resin mixture including cycloaliphatic epoxy resins, a cycloaliphatic anhydride hardener and a catalyst. WO2009/089145 further discloses adding two or more epoxy resins and a cycloaliphatic anhydride hardener to form a curable composition, wherein the epoxy resins include at least one cycloaliphatic epoxy resin and epoxy novolac resin; and thermally curing the composition at a temperature of at least 150° C. to result in a thermoset resin having a glass transition temp of at least 210° C. However, WO2009/089145 does not disclose any thermal, mechanical or chemical resistance improvements. The disclosure of WO2009/089145 illustrates a known approach to enhance the Tg of a mixture of epoxies (cycloaliphatic plus high functionality epoxy novolac resins) with cycloaliphatic anhydride and an accelerator. However, the WO2009/089145 does not disclose compositional ranges that include off-stoichiometric ratios of curing agent to epoxy. WO2009/089145 utilizes an anhydride curing agent, such as nadic methyl anhydride, which is known to provide higher Tg if cured using certain curing conditions. Multifunctional resin was used in WO2009/089145 to increase the Tg, but the mechanical properties disclosed using anhydride curing agent are undesirably low.
U.S. Pat. No. 4,540,750, which is hereby incorporated by reference in its entirety, discloses a method for making an adduct using diethyltoluenediamine (DETDA). The U.S. Pat. No. 4,540,750 discloses the DETDA as a polyamine curing agent. Since epoxy reactions with DETDA are very sluggish, the U.S. Pat. No. 4,540,750 discloses the use of an adduct to increase the reactivity of DETDA. In example A of this invention, 1-methyl imidazole (AMI-1) was used with DETDA adduct; however, the thermal properties, including Tg, resulting from the mixture were undesirably low compared to the examples without use of imidazole and did not disclose the effect on mechanical properties.
U.S. Pat. No. 4,528,308, which is hereby incorporated by reference in its entirety, discloses epoxy resin formulations, particularly curing agent formulations where large amounts of imidazole are used and polyetheramines were added to enhance the flexibility/toughness. 1-methyl imidazole (AMI-1) and 2-ethyl 4-methyl imidazole (EMI-24) are disclosed as being utilized at high loading in the formulation. The imidazole is used to initiate the cure of the epoxy resin and a very low amount of polyether amine is added back to the formulation to improve the flexibility (i.e. % elongation). In the U.S. Pat. No. 4,528,308, the imidazole is used as the primary reactant while the polyether amine is used as the secondary to improve the flexibility/toughness of the system. U.S. Pat. No. 4,528,308 does not disclose glass transition temperature or chemical resistance improvements.
U.S. Pat. No. 5,672,431, which is hereby incorporated by reference in its entirety, discloses epoxy resins and, more particularly, to epoxy resins incorporated with an imidazole accelerator in combination with chromium acetylaacetonate (Cr (acac)3). The U.S. Pat. No. 5,672,431 discloses a solid amine hardener of 4,4′-diaminodiphenyl sulfone (DDS) with imidazole and tertafunctional resin. The resultant cured resin had an increased fracture toughness, but the glass transition temperature dropped significantly. The U.S. Pat. No. 5,672,431 discloses a multifunctional resin in combination with aromatic amine to achieve a higher fracture toughness and interlaminar shear properties, but with undesirably low strength and Tg.
Epoxy compositions, epoxy products and epoxy composite products having desirable physical, thermal and chemical properties not suffering from the above drawbacks would be desirable in the art. These needs are addressed by the embodiments of the present invention as described below and defined by the claims that follow.